The present invention relates to a cooling water expansion tank having a level-fixing device that is a positive pressure valve which controls a vent conduit which, starting from a tank top, emerges into the filling duct of a filling stub-pipe which can be closed by a cap. The filling duct has, for limiting the level, a lower edge located in the internal space of the tank and at a distance from the tank top, and sealed boundaries located between the cap and the tank. These sealed boundaries are provided in the vent path and shut off the vent conduit from the tank internal space and from the atmosphere when the cap is closed.
A cooling water expansion tank of the above-described type is described in German Patent Document DE-PS 30 07 272. In this tank, an overflow conduit leading to the outside is located on a filling pipe which can be closed by a cap to fix the level in the expansion tank. The filling pipe is attached to a side wall of the expansion tank. During driving operation, the cooling water becomes heated and expands into the air space, located above the water level, of the closed expansion tank. The heated cooling water is then held back from running out via the overflow conduit by a seal between the cap and the filling pipe. When the cap is opened, however, the seal is removed so that the cooling water can flow away unhindered via the overflow conduit until the cooling water level has fallen below the lower edge of the overflow conduit. This has the disadvantage that when the cooling water has previously expanded over the lower edge of the overflow conduit, cooling water is extracted from the expansion tank and therefore from the cooling circuit on every opening procedure and this reduces the cooling capability. In addition, the adjustment of the level on filling the expansion tank leads to a loss of cooling water, requiring continuous filling during the adjustment of the level until the cooling water emerges from the overflow conduit.
An object of the present invention is to provide a cooling water expansion tank in which the cooling water does not run out when the filling pipe is opened after a driving operation or when the expansion tank is filled.
This and other objects are achieved by the present invention which provides a cooling water expansion tank arrangement having a tank with a tank top and an internal space, a level-fixing device including a vent conduit and a positive pressure valve, a filling stub-pipe having a filling duct with a lower edge located in the internal space of the tank and at a distance from the tank top, a cap which closes the filling stub-pipe, a vent space between the filling stub-pipe and the cap, a vent conduit between the vent space and the tank top, the vent conduit receiving a positive pressure valve, a first sealed boundary between the vent space and the atmosphere and a second sealed boundary between the vent space and the internal space of the tank, the sealed boundaries limiting the vent space. The sealed boundaries shut off the vent conduit from the tank internal space and from the atmosphere when the cap closes the filling stub-pipe. The first sealed boundary is open and the second sealed boundary is closed when the cap is only partially opened.
The opening characteristic of the positive pressure valve is such that it only opens when the pressure occurring on filling the expansion tank slightly exceeds a positive pressure specified at the positive pressure valve. Consequently, the expansion tank can be filled up to the lower edge of the filling pipe, as far as the upper edge of its mouth in the case of a filling pipe which protrudes obliquely. This ensures that the level is fixed and creates an air space with a slight positive pressure located above the liquid surface. The cooling water can expand into this air space when it becomes hot during driving operation. If further filling occurs, the level rises in the filling pipe only.
For an embodiment of a filling pipe having low design height attached to the side of the expansion tank, it is possible that in the expanded condition, the cooling water level in the expansion tank is so far above the overflow edge of the filling pipe that cooling water runs out of the filling pipe when the cap is removed--despite venting of the tank--because of level equalization between the expansion tank and the filling pipe. In order to avoid this in the compensation tank, an embodiment of the present invention provides an overflow edge on the filling pipe, this overflow edge being at least at the same height as the cooling water level at maximum expansion height.
In another embodiment of the present invention, the filling pipe is located in the top of the expansion tank. A wall piece of the filling pipe protruding out of the tank forms, together with the tank top, an annular disk and a pipe piece, a hollow space which has, as vent conduit, openings to the internal space of the tank (in the top) and to the inside of the filling pipe. Because of the arrangement of the filling pipe in the tank top and the structural utilization of the wall piece and the tank top to form a vent conduit, complicated external conduit lines are omitted.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.